The present invention relates to a method of manufacturing a golf ball having an inner sphere composed of a core and one or more envelope layer enclosing the core, and having a cover which encloses the inner sphere. More specifically, the invention relates to a method of manufacturing a golf ball which includes injection molding the inner sphere, then, without completely releasing the spherical material from the mold, successively molding the cover.
Generally, to mold a golf ball core, unvulcanized rubber is formed into a slug of a fixed weight by way of an extrusion step. The slug is then inserted into a vulcanizing mold, where it is subjected to vulcanization under pressure, thereby giving a core made of a rubber material in the form of a sphere. To produce via pressure vulcanization a rubber sphere of two or more layers, an envelope layer-enclosed rubber sphere can be obtained by injection-molding one envelope layer about the core while holding the core with pins or the like, then cooling and demolding. If two or more envelope layers are to be formed, the above process may be repeated to form a plurality of envelope layers.
The following method is generally used to subsequently render the above rubber sphere into a golf ball. That is, the core or the inner layer of rubber enclosed by an envelope layer described above is set within a mold having dimple-forming features. A cover material is then injection-molded over the envelope layer while the envelope layer is held by support pins or the like. After cooling, the molded body is removed from the mold, then stamped and clear-coated.
In the manufacturing method described above, because the sphere is supported inside the mold cavity with support pins, sphere eccentricity tends to arise. To address this problem, JP-A 51-118535 and JP-A 8-322966 describe methods which involve forming the cover on the surface of the sphere one hemisphere at a time, and thus do not require support pins.
However, in these prior-art golf ball manufacturing methods, forming two or more cover layers over a core or an inner layer of rubber enclosed by an envelope layer requires the use of two or more different molds having cavities that correspond to the cover layer thicknesses. As a result, when the molded sphere is transferred from one mold to another mold and a cover material is injection-molded thereon, dust and debris tend to adhere to the surface of the sphere, sometimes resulting in a loss in the strength of the bond between the surface of the sphere and the overlying cover.
Also, the complete formation of two or more cover layers requires, in the formation process, that hemispherical sections be formed for each cover layer, which increases the number of operations. Moreover, because the component sections are separately fabricated, the adherence of dust and debris weakens the strength of bonded areas therebetween.